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From: Attila Soki <[email protected]>
To: [email protected]
Subject: Re: unstable query plan on pg 16,17,18
Date: Mon, 23 Feb 2026 16:10:38 +0100
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>


> On 23 Feb 2026, at 10:41, Laurenz Albe <[email protected]> wrote:
> 
> On Mon, 2026-02-23 at 10:37 +0100, Attila Soki wrote:
>> When upgrading from PostgreSQL 14.4, I noticed that one of my somewhat complex
>> analytical queries sometimes gets an inefficient plan under PostgreSQL 16, 17, and 18.
>> Under 14.4, the query runs with a stable plan and completes in 19 to 22 seconds.
>> In newer versions, the plan seems to be unstable, sometimes the query completes
>> in 17 to 20 seconds, sometimes it runs for 5 to 18 minutes with the inefficient plan.
>> This also happens even if the data is not significantly changed.
> 
> This is very likely owing to a bad estimate.
> 
> Could you turn on "track_io_timing" and send us the EXPLAIN (ANALYZE, BUFFERS) output
> for both the good and the bad plan?

Hi Laurenz,

Thank you for your reply. Here are the two explains.
In order to be able to publish the plans here, I have obfuscated the table and field names, but this is reversible, so I can provide more info if needed.

plan-ok:
https://explain.depesz.com/s/hQvM

plan-wrong:
https://explain.depesz.com/s/uLvl

Best regards,
Attila

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